Theory of HFV

1
Theory of HFV

• Gas Transport Mechanisms
• Oxygenation
• Ventilation

2
HFV Gas Exchange

• Henderson first published his findings in 1915,
  assessing dead space relationship in ventilation.
• He stated, there may easily be a gaseous
  exchange sufficient to support life even when VT
  is considerably less than dead space.

3
HFV Gas Exchange

• Chang theorized that convective processes were
  more predominant with an increase in VT and lower
  frequencies. A diffusive mechanism may be more
  predominant where there is a decrease in VT and
  higher frequencies are used.

4
HFV Gas Exchange

• In the 1970s, Bunnell and his associates
  demonstrated in animals that adequate alveolar
  ventilation could be achieved with a frequency
  between 5 - 30 Hz and a VT of 20 - 25 less
  volume than anatomical dead space.
• Slutsky, et al. theorized that the gas exchange
  mechanism was caused by the coupled effects of
  convection and molecular diffusion.

5
Theory of HFV Gas Exchange

• Gas exchange is enhanced by additive mechanisms
  to the bulk flow and molecular diffusion of
  conventional ventilation.
• 6 mechanisms for gas exchange have been noted
  with high frequency ventilation

6
How does HFV work?

• Convection (Bulk Flow) Ventilation
• Even with small tidal volumes, direct alveolar
  ventilation occurs to short path length units
  that branch off of the primary airways.

7
How Does HFV Work?

• Taylor Dispersion
• Convective flow superimposed on a diffusive
  process, results in increased dispersion of the
  tracer molecules.
• The high velocity spike of gas moves down the
  center of the tube, leaving the molecules on the
  periphery unmoved. Gas diffuses evenly through
  the tube when flow stops.

8
How Does HFV Work?

• Asymmetrical Velocity Profile
• During inspiration, the high frequency pulse
  creates a bullet shaped profile, with the
  central molecules moving further down the airway
  than those molecules found on the periphery of
  the airway.
• 2 Steps Forward

9
How Does HFV Work

• Asymmetrical Velocity Profile
• On exhalation, the velocity profile is blunted so
  that at the completion of each return, the
  central molecules remain further down the airway
  and the peripheral molecules move towards the
  mouth of the airway.
• 1 Step Back

10
How Does HFV Work?

• Pendeluft
• At high frequencies, distribution becomes
  strongly influenced by time constant
  inequalities. Gas from fast units (short time
  constants) will empty into the slow (long time
  constants) units.

11
How Does HFV Work?

• Cardiogenic Mixing
• The heart beat adds to the the peripheral gas
  mixing.

12
How Does HFV Work?

• Molecular Diffusion
• Is felt to be one of the major mechanisms for gas
  exchange in the alveolar regions.
• It is responsible for the gas exchange across
  the AC membrane and also contributes to the
  transport of O2 and CO2 in the gas phase near
  the membrane.
• This may be due to the increased turbulence of
  molecules.

13
Theory Of Operation

• Oxygenation and CO2 elimination have been
  demonstrated to be decoupled with HFOV.

14
Oxygenation

• Oxygenation is primarily controlled by the Mean
  Airway Pressure (Paw) and the FiO2 for Diffuse
  Alveolar Disease (DAD).
• Ventilation is primarily determined by the stroke
  volume (Delta-P) or the frequency of the
  ventilator.

15
Oxygenation

• The Paw is used to inflate the lung and optimize
  the alveolar surface area for gas exchange.
• Paw Lung Volume

16
Oxygenation

• The Paw is used to inflate the lung and optimize
  the alveolar surface area for gas exchange.
• Paw Lung Volume

17
Oxygenation

• Paw is created by a continuous bias flow of gas
  past the resistance (inflation) of the balloon on
  the mean airway pressure control valve.

18
Oxygenation

• The Paw of the oscillator without the piston
  moving is a TRUE CPAP system.
• The Paw is changed by adjusting the bias flow or
  the inflation of the balloon control valve (Paw
  Adjust).

19
Oxygenation

• CLINICAL TIPS-
• Must have adequate MAP and hemodynamic
  performance. Perfusion must be matched to
  ventilation for adequate oxygenation.
• Chest x-rays and oximetry are necessary.

20
Oxygenation

• CLINICAL TIPS-
• PVR is increased with either atelectasis (Loss of
  support for extra-alveolar vessels) or over
  expansion (Compression of alveolar capillary
  bed). The lung must be recruited, but guard
  against over expanding.
• Utilize x-rays and oximetry to wean Paw when
  rapid improvement in compliance yields lung
  overexpansion.

21
Oxygenation

• For a DAD process, increase the Paw to achieve
  adequate arterial oxygenation (88 - 93
  SaO2).
• Increase the Paw in 1 - 2 cm increments every 5
  - 10 minutes to achieve adequate oxygenation as
  reflected by the oximeter.

22
Oxygenation

• In ALS, use a lower Paw and Delta-P to minimize
  further injury and allow the leak to seal
• Low Lung Volume Strategy with Right Posterior
  Hemi-diaphragm at 7 - 8 rib level expansion for
  neonates.
• Use higher FiO2s and less than optimal ABGs
  PaO2 in the 50s with acceptable hypercarbia and
  pH gt 7.25

23
Oxygenation

• Maintain the Paw and decrease the FiO2 until it
  is at 60 or lower.
• Re-check a CXR for lung volume
• If the diaphragm is between 8 and 8-1/2, continue
  decreasing the FiO2.
• If the diaphragm is between 9 and 9-1/2, decrease
  the Paw 0.5 to 1 cmH2O
• Continue weaning FiO2

24
Ventilation

• Controlled by the movement of the pump/piston
  mechanism.
• Alveolar Ventilation during CMV is defined as
• f x Vt
• Alveolar Ventilation during HFV is defined as
• f x Vt 2
• Therefore, changes in volume delivery (as a
  function of Delta-P, Frequency, or Insp. Time)
  has the most significant affect on CO2
  elimination.

25
Ventilation

• Primary control of CO2 is by the stroke volume
  produced by the Power Setting
• The amplitude or Delta-P measurement displayed on
  the front panel is produced by adjustment of the
  Power Setting

26
Ventilation

• The amplitude is created by the distance that the
  piston moves, resulting in a volume displacement
  and a visual CHEST Wiggle.
• It may also be described as the peak-to-trough
  swing across the mean airway pressure.

27
Ventilation

• Secondary control of PaCO2 is the Frequency set.
• If the Power controls the force with which the
  piston moves, the Frequency controls the time
  allowed (distance) for the piston to move.
• Therefore, the lower the frequency setting, the
  greater the volume displacement, and the higher
  the frequency setting, the smaller the volume
  displacement.

28
Ventilation

• Recommended Guidelines for initial Frequency
  Setting (May be disease dependent)
• lt 2000 gms 15 Hz
• 2 - 12 kg 10 Hz
• 13 - 20 kg 8 Hz
• 21 - 30 kg 7 Hz
• gt 30 kg 6 Hz

29
Frequency

• Frequency MAY or MAY NOT have to be adjusted from
  the initial setting
• Frequency is not weaned as is done with CMV
  (decreasing frequency with HFOV increases
  ventilatory support).

30
Ventilation

• The Inspiratory Time also controls the time for
  movement of the piston, and therefore assists
  with CO2 elimination.
• Do Not manipulate the I-Time for frequencies of
  10 - 15 Hz. This may potentially increase
  inadvertent gas trapping.
• Increasing I-Time is used in larger pediatric
  patients as the third maneuver to control CO2
  elimination.

31
Ventilation

• The ET tube acts as a filter to the pressure
  wave, attenuating the pressure as great as 90
  with a 2.5 ET tube. The larger the tube the less
  attenuation.
• The amplitude shown on the LED read out is
  measured within the circuit
• HFOV is considered a VERY GENTLE form of
  ventilation.

32
Ventilation

• Distal amplitude measurements with alveolar
  capsules in animals, demonstrate it to be greatly
  reduced or attenuated as the pressure traverses
  through the airways.
• Due to the attenuation of the pressure wave, by
  the time it reaches the alveolar region, it is
  reduced down to .1 - 5 cmH2O.

33
Ventilation

• Bias Flow produces the Paw of the system, and
  also helps to flush the CO2 that is actively
  pulled back into the circuit during the
  expiratory phase.
• If decreasing the PaCO2 is difficult, increasing
  the bias flow through the system may be helpful.

34
Clinical Tips

• If increasing the amplitude has no net affect on
  decreasing the PaCO2, consider decreasing the
  Frequency setting by 1 Hz at a time.
• If there is minimal CWF, and the PaCO2s are too
  low, consider increasing the Frequency setting by
  1 Hz.
• With cuffed ET tubes, minimally deflating the
  cuff may allow airway wall CO2 to exit the
  circuit at the tip of the tube.

35
THEREFORE

• If the Paw CPAP
• And the Amplitude Chest Wiggle
• Then HFOV can be defined as
• CPAP with a Wiggle!
• Not nearly as complicated a concept as some try
  to make it!

36
SUGGESTED READING

• Chang HK. Mechanisms of gas transport during
  ventilation by HFOV, Brief Review, J Appl
  Physiol, 1994
• Schindler M, et al. Effect of Lung Mechanics on
  Gas Transport During HFO. Pediatric Pulmonology,
  1991
• SensorMedics Critical Care, Operators Manual for
  the 3100A, 1995